The newest version of Growl is brand new, written from the ground up for OS X Lion, and contains a number of new features. The coolest is definitely the new history feature. Now, Growl sits in your menu bar, and logs any notifications you get while you're away, so you can see them all when you get back (and even search through your history for old notifications). It's also no longer free, but it's still well worth the price at a mere $1.99 in the Mac App Store. Hit the link below to check it out.
Growl is a $1.99 download for Mac OS X only. Windows users can check out the still-free Growl for Windows.